What caused piston to burn?

[hookedonjc]

I have the 120 HP 1990 Evenrude that I am rebuilding myself and need to know what could have caused the problem of burnning a piston to begin with. All seemed to be running good water pump good Oil was OK I belive because it goes through the VRO and mixes before getting to the carbs. Only had one piston bad. I did notice the piston heads were kind of black on top but not real bad. I was told carbon build up could have been the problem . does this sound possible?<br />Thanks <br />Hooked

 

[Hooty]

Re: What caused piston to burn?

Several things can cause pistons to burn and it's very important to find out what caused that one to burn so the next one doesn't. <br />Carbon accumulation is one cause. The carbon deposit gets red hot and predetonates the fuel/air charge, which in turn melts a hole in the piston. Are the rest of the pistons carboned up? <br /><br />c/6<br />Hooty

 

[walleyehed]

Re: What caused piston to burn?

I would be pulling the carb from the cylinder with the burnt piston and pull the main jet and see if it has something in it....I'll bet money it does....

 

[BRIAN03]

Re: What caused piston to burn?

Heat Alu. melts at 1300 Degrees.

 

[hookedonjc]

Re: What caused piston to burn?

There seems to be thin black layer on the top of each piston I didnt know if that was normal. I dont know much abut the carbs and checking for pluggage in the jets How do I check this? I was planning on putting a kit on each carb would this help if the carb was the problem?<br />Thanks for Info<br />Hooked

 

[OBJ]

Re: What caused piston to burn?

Hookedonjc....get a service manual and follow the steps for carb rebuild. It's a real good idea to rebuild the carbs with an over haul.<br /><br />But do get the manual....it'll pay for itself.<br /><br />Good Luck!

 

[jim dozier]

Re: What caused piston to burn?

My first guess would be a lean condition on that cylinder. If it runs lean it not only gets hotter and is more likely to cause detonation but the oil is reduced in the lean cylinder causing increased friction and heat. What did the spark plug on that cylinder look like? Was it lighter colored than the others with evidence of a lean condition? I agree with Walleyehed, check the main jets on the carb for that cylinder. If you haven't got one by now, go to the bottom on this webpage and order a manual.<br /><br />Although excess carbon can cause pre-ignition and detonation, some black carbon on the piston tops is normal for a two stroke. It usually becomes a problem when it starts to clog the exhaust ports and sticks the piston rings. It doesn't sound from your description like anything unusual.

 

[Spidybot]

Re: What caused piston to burn?

Agree with the lean theory.<br /><br />By description it sounds just like my engine.<br /><br />A 16 valve, 4-cyl, 2 litre 185hp car engine that blew a piston without any warning.<br /><br />EFI, as all modern designs.<br /><br />Upon taking it apart (nasty, by the way), I couldn't find any obvious reason but further investigation discovered a defective injector on that particular cylinder. It did supply fuel, but not enough. It ran well - not a miss (it blew in the middle of passing a roundabout at low rpm) so the fuel was enough to run on yet too little to cool the piston top.

 

[catfish1]

Re: What caused piston to burn?

i agree with the carb (jet) problem and getting a repair manual.

 

[Hooty]

Re: What caused piston to burn?

I agree, it could be caused by a lean charge but I'd expect some serious cylinder scoring due to lack of lube and the burn in the piston next to the exhaust port. Where'd it burn?<br /><br />c/6<br />Hooty

 

[ted655]

Re: What caused piston to burn?

Would the sparkplug on that cylinder be different in appearence?

 

[OBJ]

Re: What caused piston to burn?

Hooked....check the piston rings on the piston in question for stuck rings. This could result in blow by and effectivly cause the problem your descibing. By the way, how long have you had the engine and was decarb a part of the maint.? And I really havn't seen in your post a real description of the damage. Was the top of the piston burnt/melted and was there any cylinder wall scoring.<br /><br />Maybe I missed something somewhere....(senior lapse) I dunno.

 

[hookedonjc]

Re: What caused piston to burn?

Thanks for the replys<br /> I have removed the bad piston and what I am seeing is a thin layer of aluminum all through the cylinder maybe a little heaver on the bottom which looks to be half intake area and half exaust area. Overall I dont see any scaring on the walls just alluminun stuck to the walls. This can be honned I hope? Also the plug on just this cylinder is coated with alluminun splater no sign of any oil. The other three seem to be OK like they were firing ok. with a little oil residue on the head caps where the plugs screw in. Also the rings are completly imbeaded in the piston with a piece about one inch long broken off it fell out when I pulled the cap off. What is the best way to remove the piston off of the rod arm. Looks like it could be a bear.<br />Thanks <br />Hookedonjc

 

[OBJ]

Re: What caused piston to burn?

Hi Hooked....what's the top of the piston look like? Melted?

 

[walleyehed]

Re: What caused piston to burn?

Ya got a plugged jet......

 

[Hooty]

Re: What caused piston to burn?

Roger the plugged jet. When you say rod arm, are you talking about the wrist pin? What does your manual call it?<br /><br />c/6<br />Hooty

 

[hookedonjc]

Re: What caused piston to burn?

Hey Guys<br /><br /> I got plenty of Questions now. After pulling the other three pistons. I have discovered slight scaring on each about a half inch wide the full length of the piston. Now Question one. even though only one piston had the most damage does this mean I might of had a oiling problem after all. And question two can I still use the three scared pistions with new rings? The rings move freely on the piston and dont seem to have major damage. The walls of the cylinder look to be Ok with a little honning. Looking at the engine from behind the one with the major damage is on the top left.<br />Thanks <br />Hookedonjc

 

[rodbolt]

Re: What caused piston to burn?

hello<br /> bore all 4 and go with the next oversize. I have seen many times people try to hone and go. usually works for a limited time. the piston to wall clearence is about .004" max. after honing and with normal skirt wear, much less visible damage you will have a fresh rebuilt wore out motor that coughs at idle.if the VRO is over 6 years old then replace it. if the oil lines are hard as a rock then replace the pickup assy. its about 79 dollars. always use new rodbolts. they can fail if reused due to the high stress on such a small bolt.AP makes a bearing kit. if you have to cut corners reuse the rod and upper and middle crank bearings. replace the wristpins,pin bearings, pistons, rings, rodbolts and bottom main at a bare minimum. if you cant replace the recirc check valves at least clean and test them. go thruogh the carbs carefully. replace all the fuel lines as they can break apart on the inside with age. the list goes on and on. I wont cut corners anymore as if its worth my time then its worth doing it right but I understand sometimes the wallet cant pay the first class fare.<br /> good luck and keep posting

 

[Cricket Too]

Re: What caused piston to burn?

What is the best way to clean and test the recirc valves? Thanks.

 

[Dhadley]

Re: What caused piston to burn?

You mentioned the rings being "embedded" in the grooves on the other pistons and a broken ring on the worst piston. Are the other rings stuck from a carbon build up or from scoring on the piston? All this sounds like a typical coking problem which is likely to be related to higher combustion temps.<br /><br />What spark plugs were you using? What was the top rpm when the motor did run right? What fuel were you using?<br /><br />Good luck!